This invention relates to a method of producing a catalyst for the catalytic oxidation of carbon monoxide and hydrocarbons in exhaust gas of internal combustion engines, which catalyst is of the type comprising a platinum group metal distributed upon a carrier of active alumina.
In the art of preventing the air pollution caused by the exhaust gas of internal combustion engines, particularly, of automotive use, the removal of carbon monoxide (CO) and unburned hydrocarbons (HC) from the exhaust gas is in many cases accomplished by catalytic oxidation. A catalyst for this use is required to be effective over a wide range of reaction condition and have a good durability because the catalyst has to be maintained in the exhaust system of the engine for a considerably long period of time while the exhaust gas has considerably high temperatures and exhibits frequent and indeterminable fluctuations in its temperature, composition and flow rate according as the operating condition of the engine or automobile varies. To meet these requirements, the most widely used catalytic substance at present is the metals of platinum group, either singularly or in combinations. A practical catalyst is given by impregnating or distributing the catalytic metal(s) upon a support or carrier of ceramics material in a form having a large specific surface area. Typical examples of such a material are active alumina such as gamma alumina and cordierite with an alumina coating. In a usual method of producing this type of catalyst, the carrier is impregnated with a solution of a thermally decomposable compound of the catalytic metal such as chloroplatinic acid or palladium chloride and then the impregnated carrier is baked in air or hydrogen to cause pyrolysis of the impregnated compound.
Conventional catalysts of this type are not yet fully satisfactory either in the conversion ability for HC on in its durability or ability of long retaining the catalytic activity. As an additional problem, these catalysts exhibit catalytic activity also on the oxidation of sulfur dioxide SO.sub.2 in the exhaust gas to sulfur trioxide (anhydrous sulfuric acid) SO.sub.3 : this oxidation reaction is an unwanted one. This activity is particularly notable on catalysts containing platinum or palladium which exhibits a high conversion ability for CO and HC.
Recently the emission of sulfur oxides, particularly SO.sub.3, from internal combustion engines has become a matter of serious concern. The presence of sulfur oxides in the exhaust gas of course originates in the presence of sulfur matter in gasoline. Although current techniques for the production of gasoline include sulfur removal procedures, it is practically impossible to provide a strictly sulfur-free gasoline. Commercially available gasoline contains, on the average, no less than about 0.01-0.02 Wt% (on the basis of S) of sulfur matter, so that the combustion gas of gasoline inevitably contains SO.sub.2.
Since the exhaust contains about 10% of water (steam), if SO.sub.2 is converted into SO.sub.3 by the action of an oxidation catalyst disposed in the exhaust system, there is a strong possibility that SO.sub.3 reacts with steam, resulting in the emission of sulfuric acid mist into the atmosphere. Even when SO.sub.3 is discharged into the atmosphere without undergoing such reaction in the exhaust system, the same reaction tends to occur upon rapid cooling of the moisture-containing exhaust gas at the outside of the exhaust outlet.
The above described process may be expressed as follows, wherein the sulfur matter contained in gasoline is simply represented by S. ##EQU1##
There is a demand, therefore, for a catalyst which exhibits high catalytic activity on the oxidation of CO and HC but does not significantly catalyze the oxidation of SO.sub.2 to SO.sub.3.